A monolithic metal catalytic converter is usually preferred to the supported-catalyst type of converter because of the high surface-area:mass ratio. It is therefore possible with a monolithic metal catalytic converter to achieve excellent results and considerable treatment with a very small unit.
The supported-catalyst converters have, however, the advantage that they are considerably more robust. In such arrangements the catalyst, normally a metal oxide, is carried on a ceramic substrate so that the unit can be made extremely robust. As the catalytically active metal used in the monolithic metal catalytic converters is extremely expensive, such metal is normally used in the form of very thin foils. Thus a monolithic metal catalytic converter is normally relatively fragile. Furthermore when such a converter is used as a catalytic muffler in a motor vehicle it frequently becomes internally loose so that the parts rattle and, indeed, frequently form a sort of resonator that completely impairs the muffler function.